1. Field of the Invention
The present invention relates to a process and an apparatus for the dry forming of a material web from a long-fiber material, wherein fibrous material is blown into a forming space to form a porous material web on a wire passing through the forming space.
2. Description of the Related Art
In dry forming processes, such as dry paper-making machines, special forming parts for the screening and processing of the fibrous material are employed, wherein a uniform material web is produced on the wire by employing and regulating various mechanical screens, cleaning and mixing devices, and air currents. Thereafter a bonding agent is sprayed onto the material web, and the web is transported into a heating zone wherein the bonding agent melts and adheres to the fibers, bonding them together into a firm paper product.
The number and shape of perforations in the mechanical screens, such as forming drums, as well as the shape and other similar properties of the screens employed in the forming parts referred to above are of crucial importance for the quality of the material web and thereby for the final product. An inherent quality in the screens is that the higher the average fiber length in the raw material, the more critical the selection of a correct screen and correct use of the screen. This is a matter of current interest particularly in view of the present-day dry-formed products based on long synthetic fibers. While the average length of wood fibers is 2 to 6 mm, synthetic fibers may in principle have an infinite length, but with the present technology it should be possible to dry-form webs of synthetic fibers having a maximum length of 20 to 25 mm. However, this requires a fairly complicated forming machinery having a manifold forming unit and complex tubing and recycling equipment. In this regard, reference is made to European Patent 188 454.
One concrete set of problems is presented by the manufacture of GMT (Glass Mat Thermoplastics) products. The car industry, in particular, currently uses more than 25,000 tons of GMT parts per annum, and the consumption is forecast to increase to 60,000 by 1995. The advantage of GMT products over thermosetting plastics is the possibility of reusing the products. Glass fiber is normally used as reinforcing fiber, and polypropylene is used as the raw material for the matrix.
The strength of GMT products is influenced for instance by the proportion of reinforcing fibers in the product, the length of the reinforcing fibers, and the surface finishing thereof. With a 30% glass fiber content, the tensile strength obtained for the product is approximately 70 MPa/mm.sup.2. With rock fibers, i.e. mineral fibers, a tensile strength of 30-40 MPa/mm.sup.2 can be obtained, respectively. As research proceeds and special materials are employed, the strength values can be expected to further increase significantly. The GMT product range comprises for instance in the car industry bumpers, seats, control panels, etc.
The GMT production processes currently employed are based on coating a material web with a matrix-forming substance (Continuous Melt Impregnation Process) or on laying a material web in a bonding agent suspension (Continuous Slurry Deposition Process). Modifications of these, as well as totally new processes are being developed continually as the demand increases and the production technology is mastered. However, in all GMT processes at least the forming of the reinforcing fiber component into a material web of a uniform quality is necessary. When the glass fiber length is in the order of 50 mm, even up to 60 mm, it is obvious that conventional dry forming parts are not capable of adequate processing of the fibers. It has been found that enlarging the perforations in a screen member in principle improves the screening of long fibers onto the material web, but when the perforations have sufficient size, the screen loses its screening and distribution capability completely. Therefore, the forming technology of a material web must be developed starting from a totally new basis. In GMT products, the fiber length is not an end in itself, but the strength and bonding properties determine the minimum lengths of the fibers employed. It is obvious that very short fibers cannot be employed irrespective of their possible strength, since they do not extend to sufficiently many points of contact, i.e. bonding points, with other fibers in order for the bonded product to have sufficient strength. Thus it can be assumed that the average length of the fibrous material to be formed into a material web, or of a fiber component therein, is at least about 20 mm.